The p53 tumor suppressor pathway is inactivated in most forms of human cancer. In normal cells, p53 is maintained at low levels by ubiquitylation and subsequent proteosome-mediated degradation. In addition to ubiquitylation, p53 is regulated by other forms of post-translational modification, including phosphorylation, acetylation and methylation. These modifications regulate a variety of p53 functions, including protein half-life, DNA binding capacity and cofactor interaction. The elimination of p53 function i critical during human carcinogenesis because the normal role of p53 is to induce either cell cycle arrest or apoptosis when a cell sustains genetic damage. As the central regulator of apoptotic cell death, the p53 pathway is of critical importance for chemotherapeutic strategies that induce genotoxic damage. Understanding the key events in the p53/apoptosis pathway is therefore of broad clinical significance. We have demonstrated that p53 is acetylated at a unique site, K120, within the DNA binding domain, by the MYST family enzymes hMOF and TIP60. Acetylation at K120 occurs rapidly after DNA damage and several lines of evidence show this is essential for p53-dependent apoptosis. Further, our empirical studies show that the acetyl-K120 isoform of p53 is enriched at pro-apoptotic target genes, but not cell cycle arrest targets. In addition, conservative mutation of the acetylation site to arginine (K120R) inhibits apoptosis, without blocking G1 arrest. Remarkably, the inactivation of p53 by K120R mutation has been reported in a small number of human tumors, and both the enzymes that catalyze K120 acetylation (i.e. hMOF and TIP60) are lost in human tumors as well. These observations suggest that the activation of the K120-acetylation pathway may be an important tumor suppressor mechanism. Our current model suggests that MYST family proteins acetylate p53 at K120 after DNA damage and that acetylation then selectively enhances transcription of pro-apoptotic target genes by p53. Using recently developed tools and methodologies, we have generated genome-wide data sets that will allow us to identify all loci bound and regulated by the Ac-K120 isoform of p53. The studies proposed here will provide a thorough understanding of the role played by the K120 acetylation pathway in p53 function. Our ultimate goal is to gain sufficient understanding that we will be able to manipulate this pathway in a predictable manner in order to selectively trigger apoptosis in human cancer cells.